This invention relates generally to clutches. More particularly, it relates to a clutch for use in effecting locking engagement between a drive axle and a wheel of a four-wheel drive vehicle upon the application of power to the drive axle. The clutch automatically effects disengagement upon cessation of the application of power to the drive axle, together with a direction reversal.
Heretofore, various mechanisms have been used for engaging a drive axle with its associated wheels in a four-wheel drive vehicle. One such mechanism in the form of a wheel hub normally is disengaged to allow the wheel to rotate independently of the axle. This requires that the operator lock a pair of hubs manually to engage the drive axle and wheels, and to unlock them manually to disengage.
Another such mechanism provides an overrunning clutch which engages automatically when power is applied to the drive axle and when operation is in the drive mode. However, such an overrunning clutch inherently disengages upon operation in the coast mode. In other words, the overrunning clutch engages when the rotational speed of the axle tends to exceed the rotational speed of the wheel, but disengages when the rotational speed of the wheel tends to exceed that of the axle. Such an overrunning clutch generally provides some means by which the operator may override manually to insure locking engagement between the axle and wheel.
Yet another such mechanism provides a clutch which operates in response to the application of power to the drive axle by moving pins into slots so as to engage the axle with its associated wheel. Although a mechanism of this type will effect engagement in either the drive or coast mode of operation, there is the possibility that the pins could slip out of the slots during movement between the drive and coast modes, in which case the clutch would disengage and then re-engage automatically. At normal operating speeds such disengagement and re-engagement could cause severe shocks to the clutch components and, indeed, to the entire driveline. This would result in a dangerous and possibly destructive condition. Further, in a float condition wherein the axle is rotating but no torque is transferred between the axle and wheel, an inadvertent tendency for movement between the drive and coast modes could develop. This also could cause the clutch to disengage and then re-engage, thus establishing the same dangerous condition.
U.S. Application Ser. No. 799,793, filed May 23, 1977, is directed to an automatic clutch which overcomes the deficiencies of the prior mechanisms. As disclosed therein, the clutch automatically engages a drive axle and an associated wheel upon engagement of a four-wheel drive system, maintains engagement positively in the drive and coast modes of operation as well as during the transition between drive and coast, maintains engagement positively in the forward and reverse modes of operation as well as during the transition between forward and reverse, and disengages automatically upon a slight direction reversal when the four-wheel drive system is disengaged.
A deficiency of this clutch is that it includes a ring which develops frictional drag sufficiently high to effect clutch engagement. Thereafter, this high drag continues even though it is not required to maintain clutch engagement. Thus, in order to conserve power, reduce heat, ease the problems of material selection, etc., there was still a need for an automatic clutch of this type which would develop relatively high frictional drag initially to effect automatic clutch engagement, and would develop relatively low frictional drag thereafter.
U.S. Applications Ser. Nos. 868,587, filed Jan. 11, 1978, and 27,347, filed Apr. 5, 1979, are directed to an improved automatic clutch which meets this need. The clutch includes a mechanism for developing frictional drag to effect clutch engagement. The mechanism incorporates a wrapped spring characterized as having relatively high resistance to slippage in the spring-tightening or wrapping direction, and relatively low resistance to slippage in the spring-loosening or unwrapping direction. Upon initial rotation of the driving member, the spring is caused to tighten, thereby effecting clutch engagement. Thereafter, the spring is caused to loosen, thereby reducing frictional drag to a minimum. The improved clutch also includes an actuating mechanism which subjects a movable clutching sleeve to a preload. In the event clutch engagement is blocked, the actuating mechanism can complete its cycle of operation. When the blockage is relieved, the preloaded clutching sleeve completes clutch engagement. This clutch further includes a restraining device which creates high resistance to movement of the movable clutching sleeve. This resistance is effective only during initial rotation of the driving member, thus negating the effect of any spurious force which might tend to rotate the driving member.
There remains a need in the art for an alternative automatic clutch for use in effecting and maintaining engagement between a drive axle and a wheel of a four-wheel drive vehicle upon the application of power to the drive axle. Such an alternative clutch should automatically effect disengagement upon cessation of the application of power to the drive axle, together with a direction reversal.